Electrical double layer capacitors are in practical use as backup power sources in electrical devices such as personal computers, and they are also under development as power sources of rapid discharge and large electric current such as auxiliary batteries for automobiles and the like. The electrodes of electrical double layer capacitors are also called polarizable electrodes and use activated carbon which is electrically conductive. Electrical double layer capacitors are used in various ways such as in situations of long term use with minute electric currents, situations of short term use with large electric currents, and in applications between these.
As the starting material for a carbonaceous material for an electrode, carbonized plant and animal material such as coal, coke, coconut shell char; any kind of resin such as phenol resin, furan resin, vinylidene chloride resin, etc. which have been heat-treated (dry-distilled) in an atmosphere of inert gas, and the like may be used. In the present invention, these starting materials are called carbon compounds in general, and materials obtained by dry-distilling carbon compounds are called dry-distilled charcoal.
As commonly used methods for obtaining activated carbon, methods in which activation treatments are conducted using an oxidizing gas such as steam, carbon dioxide gas, and air are known.
As an example of steam activation, there is Japanese Patent Application, First Publication, No. Hei 1-242409; as an example of carbon dioxide activation, there is Japanese Patent Application, First Publication, No. Hei 5-132377; as a combination method of air (oxygen) activation with steam and/or carbon dioxide activation, there is Japanese Patent Application, Second Publication, No. Hei 5-49606; and in addition, as an example of activation by means of hydroxides of sodium, potassium, and the like, there is Japanese Patent Application, First Publication, No. Hei 2-97414 (Japanese Patent Application, Second Publication, No. Hei 5-82324).
However, in normal activation treatments, the activation yield is of the level of 40.about.80% and the carbon loss reaches 20.about.60%. In addition, it is not possible to form pores of a uniform pore diameter.
Here, the activation yield takes the weight of carbon compounds before treatment as 100, and expresses the weight after treatment.
As electrical double layer capacitors which use carbonaceous material as electrodes, there are Japanese Patent Application, First Publication, No. Hei 1-321620 (carbon paste electrodes); Japanese Patent Application, First Publication, No. Hei 3-180013 (electrical double layer condensers); Japanese Patent Application, Second Publication, No. Hei 6-56827 (polarizable electrodes and manufacturing method); Japanese Patent Application, Second Publication, No. Hei 4-44407 (electrical double layer capacitor); and Japanese Patent Application, Second Publication, No. Hei 4-70770 (electrical double layer capacitor).
Japanese Patent Application, Second Publication, No. Hei 4-44407 discloses an electrical double layer capacitor which uses, as a polarizable electrode, an activated carbon obtained by preparing a phenol resin foam from phenol resin, polyvinyl alcohol and starch, and then conducting an activation treatment thereon. Since a porous activated carbon having a specific surface area of 2000 m.sup.2 /g or greater is obtained, it is possible to make a carbon electrode for an electrical double layer capacitor of large capacity.
Japanese Patent Application, Second Publication, No. Hei 4-70770 discloses that when the average pore diameter of an activated carbon is made 15 .ANG. or greater, a carbon electrode having good temperature characteristics is obtained.
Japanese Patent Application, First Publication, No. Hei 3-180013 discloses that by making the oxygen content of a powdered activated carbon 20.about.35% by weight, the capacitance per unit of volume is increased.
Conventionally, the electrodes of electrical double layer capacitors use powdered activated carbon base material which has been made into a paste by mixing with a sulfuric acid solution, but the contact resistance between the particles of activated carbon is large, and large electrical currents cannot flow. In addition, in the same way, when fibrous activated carbon is used, the contact resistance between the fibers is large, the density of carbon per unit of volume is small, and it is not possible to obtain large electrical currents.
The structure of the carbonaceous material can take various forms depending on the starting material and the manufacturing method.
Char and activated carbon obtained by activating char comprise microcrystalline carbon (crystallite), and carbon which takes on a chain structure. When the carbonaceous material is a nongraphitizing carbon, the crystallites take on a structure which is layered in a disorderly manner, and a wide range of pores, from micropores to macropores, are formed in the gaps between these crystallites.
The crystallites are layers of net planes of six membered carbon rings of several parallel layers, and graphite carbon which has a six membered carbon ring structure bonds using hybridized orbitals SP.sup.2. A net plane comprising six membered ring carbon is called a basal plane.
A graphitizing carbon develops crystallites by means of heating at a high temperature, and finally becomes graphite.
A nongraphitizing carbon usually contains unorganized carbon. Unorganized carbon is carbon other than graphite carbon which is chemically bonded to graphite carbon only; carbon which has a chain structure; carbon which is stuck around six membered ring carbon; carbon which is in the periphery (the prism plane) of six membered ring carbon; carbon which is held in cross-linked structures with other six membered carbon rings (crystallites), and the like. Unorganized carbon is bonded with oxygen atoms, hydrogen atoms, and the like in forms such as C--H, C--OH, C--OOH, and C.dbd.O; or is in the form of double bonded carbon (--C.dbd.C--).
When pores have a diameter of 8 .ANG. or less, they are called sub-micropores, when they have a diameter in the range of 8.about.20 .ANG., they are called micropores. Pore diameters within these spheres are approximately of the same order as the diameter of electrolyte ions, and therefore these pores are believed to take part in the formation of electrical double layers. Because present measuring techniques are unable to directly observe the pore structure of pores in the sub-micropore range, the situation at present is such that it is not possible to establish this as a general theory.
However, in conventional manufacturing methods for carbonaceous material for electrical double layer capacitors, since micropores and/or sub micropores are not sufficiently developed, the storage capacity for electrochemical energy is small, and capacitance is not sufficient. In addition, there is the problem that the carbon yield is low. In addition, efficiency for rapid discharge is unsatisfactory.